Owing to the concerns over diminishing fossil fuels and severe environmental damages caused by emissions from conversion of fossil fuel to energy, renewable clean energy has become a topic of immense interest and pressing urgency. In recent years, harvesting solar energy has received growing attention as a feasible renewable green energy solution. This may be competitive with other renewable energies such as hydro and wind powers if the efficiency of harvesting devices can be optimized and their associated implementation costs minimized. Today, solar energy conversion to electricity using silicon-based solar panels remains one of the costliest green energy approaches, despite recent plummets in solar panel costs by as much as 70%. Accordingly, the potential of using organic solar cells to reduce costs has emerged since solar cells made from organic materials may be significantly cheaper and their installation, less cost-intensive, enabling substantially reduced total costs of solar energy conversion.
One of the most studied organic solar cells is bulk heterojunction organic solar cells (BHJ-OSCs), which utilize an active layer composed of an organic electron donor and acceptor dispersion in which the donor and acceptor domain sizes are on the order of nanometers. These nanoscale domains form continuous percolated pathways for the transport of charge carriers (holes and electrons) following their dissociation from excitons after photoexcitation.
Light absorption occurs in the electron donor or acceptor domains or both, resulting in the formation of excitons, which travel to the electron donor/acceptor domain interfaces and dissociate into charge carriers. Thus, the efficiency of BHJ-OSCs is critically dependent on the efficacies of light absorption, exciton formation and dissociation as well as the transport of dissociated electrons and holes to the cathode and anode, respectively.
Presently, the most utilized acceptor material for BHJ-OSCs is fullerene derivatives (hereinafter referred to as PCBM) such as [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM), while the donor compounds are primarily p-type conjugated polymers such as P3HT, PTB7, PffBT4T-2OD, etc. For efficient energy conversion, the percolated nanoscale dispersion morphology of the donor and acceptor materials in the active layer, which promotes rapid transport of electrons and holes to their respective electrodes, is essential.
To ensure effective dispersion of PCBM acceptor and donor polymer to form proper nanoscale domain morphology for efficient carrier transport, suitable processing additives such as 1,8-diiodooctane (DIO) or 1,8-octaneditiol and the like are often added in the coating solution during solution deposition of the active layer.
The results show that the processing additive such as DIO greatly enhances nanoscale dispersion formation, leading to higher power conversion efficiency (PCE) of the resulting solar cells. However, it has been found that the nanoscale dispersions in the active layer are sensitive to their thermal environment, and morphology changes or degradation arising from thermally induced aggregation of nanoscale domains occurs as the temperature rises above room temperature. These changes have led to disruption of percolated charge transport pathways, resulting in significantly degraded PCEs.
This is particularly worrisome as the solar cells, during normal operation under direct sunlight irradiation, would be subject to temperatures significantly higher than room temperature (e.g. up to 60° C.). Thus, the performance degradation of OSCs as a result of sunlight exposure would limit their potential utility, severely hampering their practical adoption in mainstream applications. Accordingly, for practical utility of fullerene-based BHJ-OSCs, it is imperative that the thermal stability of the active layer be significantly improved in order to sustain the PCE of solar cells.
It is thus an objective of the present invention to provide an organic compound or compounds containing a fluorenone derivative structure to enhance the thermal stability of organic solar cells.
Another objective of the present invention is the provision of an active layer composition for organic solar cells wherein said composition comprises a PCBM acceptor, a donor polymer, optional processing additives, and a fluorenone derivative such that the resulting solar cells possess enhanced thermal stability.
A further objective of the present invention is to provide an organic BHJ-OSC with greatly enhanced thermal stability, and wherein its active layer composition contains a fluorenone derivative.
Citation or identification of any reference in this section or any other section of this application shall not be construed as an admission that such reference is available as prior art for the present application.